1. Field of the Invention
This invention relates to a catalyst composition having a long catalyst life and adapted to be used for the gas phase catalytic oxidation of .alpha.,.beta.-unsaturated olefinic hydrocarbons (alkenes C.sub.n H.sub.2n).
2. Description of the Prior Art
In the industrial practice of rections using catalysts, a primary problem to be solved is generally the problem of improving the activity and selectivity of the catalysts to be used. From an industrial viewpoint, it is important to maintain the catalyst life for a long time while maintaining the expected activity and selectivity of the catalyst. The term "catalyst life" referred herein means the time period beginning with the time at which the intended oxidation reaction begins and ending at the time at which the amount of the intended product is reduced below the economical level due to degradation of the activity of the catalyst during the practice of the gas phase catalytic oxidation.
In preparing the .alpha.,.alpha.-unsaturated aldehydes by oxidation of .alpha.,.beta.-unsaturated hydrocarbons, it is well known in the art that oxidation catalysts comprising molybdenum oxide are effective, and various kinds of catalysts of this type have heretofore been used. As a result of our research, however, we have found that these catalysts have a fatal defect, namely, a short catalyst life, and they cannot be used effectively on an industrial basis.
As a reason for this deficiency, it is stated at page 395 of "Practical Catalysts Classified by Reactions" (published by Kagaku Kogyosha) that "in catalysts of the molybdenum series degradation is brought about by scattering of molybdenum".
An effective solution of this problem has not heretofore been attained in the art. In this connection, Japanese patent publication No. 1848/66 proposes a process in which salts of catalyst components are fed into the reactor during the progress of the reaction, together with the starting material, to compensate for the amounts of the catalyst components consumed during the reaction. This technique is based on the assumption that the degradation of the catalyst activity is brought about by loss of the catalyst surface.
However, in such a method the reaction apparatus and operational procedure are complicated, which results in an increase in the manufacturing cost. Accordingly, such a process is not industrially convenient and it does not provide a fundamental improvement of the catalyst, per se, such as is intended in this invention.
With a view to preparing acrolein, we conducted experiments involving the reaction of oxidizing propylene with molecular oxygen using catalysts comprising an oxide of a metal, such as molybdenum, bismuth, iron or arsenic, supported on silica as a carrier. We examined the reaction with respect to the important factors of propylene conversion and acrolein selectivity, in relation to the passage of reaction time. In these experiments propylene was passed once through a catalyst layer within recycling propylene.
As a result, it was found that when the reaction is carried out at a temperature usually adopted in the art for this reaction, namely, at 370.degree. - 400.degree. C, the acrolein selectivity tends to decrease gradually even within a reaction period of not more than 1000 hours. When the reaction is carried out at a lower temperature in order to prevent the loss or degradation of the metal oxide, the conversion of propylene is reduced and the yield of acrolein and the output of acrolein per unit time and per unit weight of the catalyst is also reduced.
From the results of these experiments, it was confirmed that the tendency of degradation of the catalyst activity is conspicuous, especially in catalyst compositions comprising molybdenum oxide as one catalytic component.
Among the catalyst compositions for oxidation of .alpha.,.beta.-unsaturated hydrocarbons which have heretofore been proposed, some give very good results as regards the conversion of the starting hydrocarbon and the selectivity of the intended product. However, they are generally catalysts containing a high content of metal oxides such as 80% by weight, based on the carrier. These catalysts are very expensive and are disadvantageous to use industrially unless their catalyst life is extremely long.
We have noted that in the high temperature reaction there occurs an undesired phenomenon, namely, that molybdenum oxide in the catalyst composition is scattered and consumed, which results in a reduction of the catalyst activity. We have carried out extensive research with a view to developing catalysts capable of given a high conversion of the starting material and a high selectivity of the intended product in order to meet fully industrial demands, employing as low a reaction temperature as possible, using catalysts having a relatively low metal content, based on the carrier, and also having a very long catalyst life.
We have discovered that the configuration, structure and physical properties of silica used as a catalyst carrier are very important for maintaining the desired physical condition of the catalyst and exert a great influence on the catalytic activity of the metal oxide supported thereon, especially the catalyst life.